Druid tranquility - (segmentGranularity + tasks.partitions) data partitioning

From the docs,
Tranquility spawns one task per Druid segment. Each task is associated with a particular interval (based on your segmentGranularity) and a particular partitionNum.

I understand the use of the segmentGranularity property. druid creates one segment for time period based on this property.

Using task.partitions, i understand multiple segments are created for the same time interval.

But, How does tranquility decide on partitioning data when using task.partitions? is it arbitrary?